Anchoring system for floating drill structure



ANCHORING SYSTEM FOR FLOATING DRILL STRUCTURE Filed June 6, 1955 June 7, 1960 e. A. SCHURMAN ET AL 6 Sheets-Sheet l INVENTORS GLENN A. SCHURMAN RAY E. K/DDER BY f ' TTOS I 6 Sheets-Sheet 2 INVENTORS June 7, 1960 G. A. SCHURMAN ET AL ANCHORING SYSTEM FOR rLvATING DRILL STRUCTURE Filed June 6, 1955 GLENN A. SCHURMAN RAY E. K/DDER ITTORNEYS ANCHORING SYSTEM FOR FLOATING DRILL STRUCTURE Filed June 6, 1955 June 7, 1960 G. A. SCHURMAN ET AL 6 SheetsSheet 3 lllll [IIIIIII FlG. 3A

FIG.4

M M M5 %M Y 6 m EAK v T NW5 .T EW ,H/ an v: B W M m L l B ANCHORING SYSTEM FOR FLOATING DRILL STRUCTURE Filed June 6, 1955 G. A. SCHURMAN ET AL June 7, 1960 6 Sheets-Sheet 4 INVENTORS GLENN ,4. SCHURMAN my 5. K/DDER ITTORNEYSP" June 7, 1960 G. A. SCHURMAN ET AL 2,939,291

ANCHORING SYSTEM FOR FLOATING DRILL STRUCTURE Filed June 6, 1955 6 Sheets-Sheet 5 INVENTORS 1 GLENN A. SCHURMAN RAY E. K/DDER June 7, 1960 G. A. SCHURMAN ETAL 2,

ING SYSTEM FOR momma DRILL STRUCTURE ANCHOR Filed June 6, 1955 6 Sheets-Sheet 6 INVENTORS GLENN A. SCHURMAN RAY E. K/DDER United States Patent ANCHORING SYSTEM FOR FLOATING DRILL STRUCTURE Glenn A. Schurman, Whittier, and Ray E. Kidder, Anaheim, Calif assiguors to California Research Corporation, San Francisco, Calili, a corporation of Delaware Filed June 6, 1955, Ser. No. 513,298

1 Claim. (Cl. til-46.5)

This invention relates to offshore drilling structures, more particularly to a method and apparatus for anchoring a floating structure from which drilling or producing operations may be performed, and has for an object the provision of an improved system for anchoring a floating structure against wind, wave and tide forces so that drilling operations may be performed in deep water, say 100 feet or more, by providing an anchoring arrangement wherein both the lateral and vertical anchor lines for the drilling structure have substantially neutral buoyancy, with said neutral buoyancy being distributed uniformly throughout the length of the anchor lines or chains, so that said lines may be maintained under positive tension to limit movements of said structure under load to those permitted by the elasticity of said lines.

In the drilling of wells at an underwater site, it has been proposed heretofore that such drilling, especially in relatively deep waters, from 100 feet upward, be performed from a substantially floating platform. The primary difliculties in actually drilling from a floating platform are those of maintaining the floating structure in a relatively rigid position under varying weight loads and maintaining the .floating structure in a fixed position against the forcesof waves, tide and wind. While the .structure may be easily designed to support the weight of the derrick and several thousand feet of drill pipe required in thedrilling of deep" wells, the platform must be-designed primarily to withstand lateral and rotational movements resulting from the effects of waves, wind and tide. While it has been proposed heretofore that anchoring lines, extending laterally from the drilling platform,

be-utilized to restrain the floating platform, previously .known lateral anchoring systems have not provided adequate resistance to movement of the drilling platform under such effects'to permit drilling therefrom with a I firm connection between the underwater bottom and the platform, exceptfor the usual conductor and drilling pipes Accordingly, it has been considered more desirable to construct, where possible, platforms supported by .steel framework supported directly on the ocean bottom.

Obviously in water depth over aboutlOO feet, structural designof such platforms becomes extremely complex due to the large forces acting thereon as well as due to the major portion of the mass of the structure being supported at the. end of a relatively limber structure. This type of construction is of course comparable to an inverted pendulum and accordingly quite susceptible to earthquake damage.

One of the reasons why previously known anchoring systems have not been capable of eifectively withstanding the effects of wave,.wind and tide forces is the manner in which conventional anchor lines dispose themselves even under great tension between the anchor and the floating structure. It is well known that such anchor lines normally assume the form of a catenary curve which has little horizontal restorative force even when large 7, tensile forces are applied thereto. Anchor lines will take cry of a floating structure.

this catenary configuration in the water, whether such lines have either negative or positive buoyancy. Accordingly, previously known anchor lines have provided little more than loose moorings for a drilling structure, or working platform, against lateral or rotational forces.

In accordance with an important aspect of the present invention, there is provided a method of and apparatus for maintaining a floating structure substantially stable against large variations in the weight load on the drilling platform, while maintaining the floating structure in a relatively fixed position above the underwater drilling site, by providing anchoring lines wherein the strain thereon may be exactly balanced by a restoring force supplied by the elasticity of the lines. The entire load of the drilling platform and the drilling equipment may thus be supported by the buoyancy of the water while the floating structure is restrained against lateral and rotational movement due to the effect of WZJVBS, wind, andtide, through a plurality of laterally extending anchor lines which are of substantially uniform neutral buoyancy throughout their length. In a preferred form of the invention each individual element or segment of said anchor line is neutrally buoyant throughout its entire effective length. Further, in accordance with a preferred form of the invention, the displacing force is distributed to a pluraltiy of anchor lines having uniform,- neutral-buoyancy which surround the periphery of the floating drill platform by interconnection of at least a pair of said lines to individual underwater anchors to provide a restoration force opposing the displacement force around the floating structure, which is substantially independent of the direction of said displacement force arising from the effect of wind, wave or tide.

Further objects and advantages of the present invention will become apparent from the following detailed descrip tion taken in conjunction with the accompanying drawings which form an integral part of the present specification. 1 p p In the drawings:

Fig. l is a perspective view of a preferred form of floating drill platform anchored in accordance with the present invention to provide a stable drilling above-an underwater drilling site.

structure Fig. 2 is a side elevation view of the drilling platform shown in Fig. 1, illustrating in greater detail the method of securing anchor lines, embodying the present invention, to the floating structure. I

Figs. 3A and 3B are cross-sectional views illustrating the securing and tensioning system for the anchor lines illustrated in the embodiment shown in Figs. 1 and 2, and illustrating respectively, the hydraulic operators in their retracted and extended positions.

Fig. 4 is a cross-sectional view taken in the direction of arrows 44 in Figs. 3A and 38, with a portion being cut away through the retractable clamps to illustrate the construction thereof.

Fig. 5 is an elevational view through the operating shoes forming each clamp taken in the direction of arrows 5-5 in Fig. 4 and particularly illustrating the hydraulic operators for the clamp shoes.

Fig. 6 is an enlarged view of a portion of the uniformly distributed, neutrally buoyant anchor chains, constructed in accordance with the present invention, utilized for anchoring of the drill structure shown in Figs. 1 and'2 against lateral and rotational movement.

Fig. 7 is' a schematic plan view of a method of distributing the lateral anchoring forces around the periph- Fig. 8 is a perspectiveview of an alternative anchoring system for the floating drill platform shown in Fig.

1, which particularly illustrates another method of securirigand-positively' tensioning the anchor hnes for the t drilling structure to a predeterrninable extent. Fig. 9 is a side elevation view of a portion of the anchor lines, and an enlarged view of one of the main buoyancy pontoonsi for the floating drill structure, particularly illustrating the positive "tensioning system for therneutrally buoyantanchor'lmes; I Y 1 FigblOfis an end-elevation 'view' of afportionfof a ma-i'ri'huoyancy'ponto'on, taken in the direction of' arro'ws 10 143 in Fig.' '9. g '-PF-ig. ll is 'a-stress -diagram through a planebf symmetry, or the anchoring system illustrating the stresses onithe 'neutrallybuoyaht anchorlines ormin thef'elements' oftruss-like-structure. g p Rferi'ing-fnow to' the drawings}; and in'ipar'ticular'to 'F-ig. I 1, there is-fsho'wn a preferred arrangmenftor a floating drill structure 10 whiclf isadapt'ed' to=-support' a "derrick -1'1 a'n'dthe associated drilling 'equipmenh-illust'ra'ted I as "12 at an 'underwater diillingfsite, With the entire l6ad*of'dr illing e'qtriprhehttmd? housing=-' facihties tor personnel adapted to be* Supported by-'the buoyancy Ofthe- StI'HCtuIb 10. "The buoyancy'of structure 10is fp'rovided' by a pair ofenlarged buoyancy tanks 13 which, in the-present embodiment; are integrally formed as a portion of the cell-like base structure l4. As shown, baseld' is constructed of hollow cylinders and forms the 'foundation for' a plurality ofvertical supporting columns a In -'1acc'ordance with the presentinvention, the complete drill'structure; including'huoyant base 14, columns f--1'5 and drill structure; 10,;is-adapted to' be floated to the drilling site substantially *fully assembled. After arrival "at the 'drilling'sit the floating drill structure' is attached f-to -a plurality of neutrally-buoyant vertical anchor: chains 5 through which the entire structure "is pulled down- "war'dly intothe watertoia depth such that the entire structure is held considerably below its normalffloating anchor lines 17, which will resist displacement of platforrnli) when variations in force, "either lateral or rotational, are applied to the drilling structure, through wave, wind, or tide action. Thus, said force variations on the drilling structure may be made to act only to reduce or increase the positive tensionsin the snugged lateral anchor lines 1 7. In this manner, only relatively minor movements in the drilling-structure will beexperiencd during drilling. In actuality, these movements will be less than ,those of an equivalent fixed tower becausethe lateral anchor lincsare in effectmemb ers'of a-"tl'uss and the rigidity is. greater than-that ofa slim tall tower. As shown in Fig. 11, each neutrally buoyant vertical and diagonal anchor line, vWhile under tension, maybe considered to be a jointedelementor barin' a truss. While said truss includes a third dimensiongthedoading may be considered symmetrical about the plane of the simple truss comprised by lines 16 and 17, the ocean bottom and buoyancyqmeniber -ld. The tension 'and compre'ssion forces are as illustratedfby the arrowheads and the loads thereon represented as "the variable *wo'rk *load L" and ithe *buoyant'forceB acting onbase'l tl 7 :As particularly illustratedf floating drill-structure 10 1's -preferably-of sufficient height,-as-controlled by the length of vertical*mernbersfiSg-so that the "upper deckf-2 ll is from 30 *to 50 feet abovethe "mean surface level of the body of water," while the buoyancy tanker chamber structure- 14 is desirably from -30-to60 feet below the surface of the water. "By thiselevation oi -deck '20- and submergence er; "base 145 a small cross-sectional area-is presentedby-thestructurein thbse depthsof waterwhe're "wave and tide actions are most-pronounced. Vertical columns ,-15a're likewise desirablymade circular in crosssecti onso that'gthey present -little' resistance to water po'sition' in the body of'water and without decrease in the buofyancythereof. By this arrangemenh'there is applied "to the vertical anchor chains 46 through the weight of underwater "anchors" 18 a positive tensile stress which is predeterminably controlled so that it is at a1l'-" times greater than the load to be added to the deekofdrilling fstriig'iture -10. Thus; the ionly"change .in elevation of rilling platter-111 20, uponchange in load, will be due to *"thedecreas'ed"deformation,' -or'elastic elongation, of the j neutrally buoyant anchor chains 1'6;*as the'positive tenfsion' thereon is decreased. Thishdefletion is equal, or nearly equak-to 'thechange in h'eightbf ahiredplatfofm niid'er equal-load variations. I

v 1 While the anchoring systenit 'as thusf far-described-fare:

' :'-substantially neutral buoyancy throughout i their entire :iength. Thus, each lineimay be made to extendin substantially'a straight line between its lateral anchor419 andi its joint-like: connection'torthe -floating structure 10.

As particularly discussed above, the

"r In accordance :with'the present-invention,' -asfdescfibed above, neutrally buoyant; *later al 'anchoring lines 17 are "so'co'nstructed that they e'xte'nd in substantially straight lines; between 'lateral "anchors j19 'and their connections 40 to .thefloating structure I 10 through pontoons 13. 'j The lllngthe present embodiment; these: connections are shown 6 to bethrough thesenlargedponto'oni members13. By ithe xprovision ofgsaidsneutral buoyancy throughout :the

length of anchor, ;line :17 zthe; straight .line relationship may be applied an efiect-ive positive tension to lateral;

preferred structure of the 'lateral tanchoring-lines 17:"is 1' particularly illustratedrin'Fig. 6. rAs there shownfeach f line is provided "with" uniformly'f'dist'ributed buoyancy, fby forming each section or length ofline -1 7'.fro'm"an elongated column,- or 'pipe,"23,' having its 'oppositeendsv i closed byfl domelike connectors 24 which are welded tolthe" ends "of pipes 23. Themating ends of connectors 24 may be formed as or; chainjas indicated af' 25. By this manner of constructionfithabuoyancyftofithe "lat feralfancho'ring chains 17*is" distributed substantially ithrou'ghout thedength of fthe individual 's'egme'ntsfwith theiwalls' of pipes" 23 and the weight of end members if 24 'being1 proportioned to the volumeifof enclosed s ace "'26' within' each segment. "I'heie'ntire anchor lineiiniay :thus be made ,neutrally "buoyant throughoutiitsientire length. IAlte'rnatively, of course, the entire'chain Jor anchor '17 maybe formed asa single'elongatedisection of pipe with internal bulkheads at reasonable distances toiincrea'se strength andfimprovesafety. As shown, 7 each'line extends froma lateralfa'nchor 19 to atakeuP mechanism, such as' that illustrated in Figs. llandfzjwhich inclu'de rings 22. The manner of utilizing rings 2210 positivelyftension lines '17 will be described hereinafter. Suclia' single elongated Tsection of pipe "oficourse, 1 'bepropor'tio'ned throughout its'length toha've a'uniforntly distributed buoyancy and the lweight' component thereoi will'beisuchfthat theen'tire line'bejneutrally bfl'i'o'yant in seawater. Thus', thede'sifedfStraighthnefrelation- "ship'of lines 1718 maintained'throy'ghout itslen'g'th.

With particular rferen'ceitoitheimthod"ofqcofitrolling I the Ipositiveltension. in. both-vertical anchorjlines 16ahd lateral anchor'jlines 17th'er'e' isfilltistratdfiri'FigQZ'hn enlarged view bribe u pet amt of. both offthese anchor I lines. lA'sls-hown'ltherefliothi linesfle'rihinateuin alcylindrical section identified generally "a521, e'acliiof'which is provided with a plurality of longitudinally spaced ring members 22 surrounding the cylindrical elements 21. As further shown in Fig. 2, these cylindrical elements 21 and their rings 22 form the take-up of positive tensioning means in conjunction with the jacking arrangement particularly illustrated in Figs. 3A, 3B, 4 and 5. As best seen in these figures, ring members'22 are adapted to be engaged by a plurality of upper and lower clamping shoes 29 and 33, forming an upper clamp means 30 and a lower clamp means 31, which circumferentially surround column 21. As best seen in Figs. 4 and 5, both the upper and lower clamping means 30 and 31 are constructed of a plurality of shoe segments 29 and 33 which may be laterally extended, as shown in Fig. 5, or retracted to the position shown by the lower clamping means in Fig. 3B, and the upper clamping means in Fig. 3A. These shoe segments are adapted to be extended and retracted in response to the hydraulic cylinder and piston arrangement, specifically illustrated in Fig. 5, including piston elements 25, which remains substantially stationary while the cylinder means 27, formed within the shoe segments 29 and 33, moves relative to pistons 25 under the infiuence of hydraulic pressure supplied by lines 38 and 39.

In operation, each of the shoe segments 29 and 33 forming one of the clamping means is extended into engagement with one of the ring members 22 to secure column 21 to platform through pontoon 13, a portion of which is shown in Figs. 3A and 3B. With the lower clamp 31 in engagement with one of the rings 22A, as seen in Fig. 3A, the upper clamping means 30, including shoe segments 29 and 33, are brought into engagement with an upper clamping ring 22C. Lower clamping means 31 is then released and the pontoon 13 drawn downwardly along column 21 by lifting or pushing of the upper clamping means 30 against ring 220, through operation of the hydraulic ram means 36, to the position illustrated in Fig. 3B. By alternate extension and retraction of the shoe segments and operation of the hydraulic rams 36, pontoon 13 is drawn downwardly along column 21 and there is applied thereto and the interconnecting anchor lines 16 and 17 the desired positive tension. For a further description of the hydraulic jacking arrangement described hereinabove, reference is made to the copending application of Celestine Paul Besse, Serial No.

430,925, filed May 19, 1954 and now abandoned, which application is assigned to the assignee of the present invention.

Further in accordance with the present invention, the lateral restoring forces for the drilling structure may be peripherally distributed by interconnecting a plurality of the lateral anchoring lines 17 in the manner shown in Fig. 7. As there seen, at least two of the individual, neutrally buoyant lateral anchor lines 17 may be interconnected to each other through a sheave system, including sheaves 48 located underwater at the lateral bottom anchors 19. The advantage of the arrangement of Fig. 7 is that, irrespective of the direction of the lateral displacing forces applied to the drilling structure 10, the restoring forces on each of the neutrally buoyant anchor lines 17 is better distributed and the individual stress and elastic deformation of any one of the anchor lines is decreased to about one-half of the disturbing force on the anchoring line. he snugged in the manner described herein.

In the preferred manner of erecting the structure illustrated in Fig. l, the drilling structure 10, as mentioned above, is floated to its drilling site after at least some of the anchors 18 and 19 have been installed in the underwater bottom. v{These anchors may be secured in the underwater bottom by boring, jetting, or the like. Alternatively, some of the anchors may be of proper mass and weight to provide the desired anchoring force, but yet be movable by auxiliary lifting and towing equipment. Some of the anchors, such as 18, may even be carried with the drilling structure by the addition of Only one end of each pair of lines need 7 further buoyancy tanks to structure 10, before the arrival of the floating drilling platform, While others of the tensioning the neutrally-buoyant vertical anchor lines 16 and lateral lines 17, after initial engagement is made between said lines and their anchors, such as those identified as 18A, and main buoyancy pontoon 13 for platform 10. As best seen in Figs. 9' and 10, each of the lateral anchor lines, whose anchors are not shown, terminates in a sheave assembly 50 which is buoyantly supported in the water by float 52. Through sheave assembly 50, the chains,such as 53, apply positive tension to lines 17 by a takeup motor (not'shown) which by means of suitable gear. reduction, applies torque to takeup sheave 54 and idler sheave 55, both positioned within pontoon 13, to draw chains 53 and lines 17 taut. Since pontoons 13 will have a diameter of several feet, the essential gearing and drive motors (not shown) may be suitably disposed and operated therein. Control of the tension applying means may be located either within the pontoon, or'remotely operated from drilling deck 20.

Vertical anchor lines 16 are arranged to be similarly snugged, in the embodiment of Figs. 9 and 10, but as shown, these lines may include a pair of vertical chains which terminate in header 57. Header 57 supports a plurality of sheaves-59, through which takeup cable 60 is suitably threaded and then connected to tension applying drive mechanisms, such as takeup or drive sheaves 62 and 63; By use of a pair of takeup sheaves, horizontal stability, during the drawdown and tensioning operation on the vertical anchor lines 16, may be maintained with reduced tension on the individual tensioning lines.

As schematically represented in both Figs. 2 and 3, the'well is drilled through conductor pipe 40, which returns drilling fluid to the elevated drilling deck 20. In relatively deep water, pipe 40 is desirably supported along its length by a stiffener arrangement 41. Device 41 desirably comprises a pair of buoyancy chambers supported from the bottom of the drilling deck 20 through cables 42. The opening, designated as 45, in the underwater buoyancy section 14 permits conductor pipe 40 to be terminated above the water, so that if petroleum production is obtained, it may be produced therefrom and yetpermit the floating drill structure 10 to be refioated, free of the restraining anchor lines, and moved to a new drilling location.

Alternatively, the pipe stiffener 41 may have a density greater than water so that through suitable connections between the conductor pipe 40 and elements 41 there may be applied a suitable stabbing force on the conductor pipe. Such an arrangement is desirable under certain circumstances due to the relative buoyancy of even fairly large diameter conductor pipe in water. Thus, the initial spudding in of the well at the underwater drilling location may be aided by application of weight supplied by elements 41 to the conductor pipe.

From the foregoing description it will be seen that there is provided a method of erecting a floating drill structure in a relatively deep body of water wherein the drilling platform is confined to a smaller amount of movement than is possible with a tower of equal height erected on the ocean bottom. The elastic deformation of our neutrality buoyant anchor lines, when formed of steel, amounts to a few thousandths of an inch per linear foot of the anchor lines, so that each anchor line is held continuously under positive tension. Through use of said uniformly distributed neutrally-buoyant anchor lines, each anchor line extends from the floating structure to the respective anchors in substantially a straight line and exerts a restoration force that is at all times equal to the tension imposed thereon by waves, wind, and tide, acting on the floating structure.

emcee-1 i7 Narious iother'modifications andnchangesgin the method :ererecting; a; stabilizedrfio'ating drill: structure:6n aibody wise variations in the manner of constructing the neuitrallyebuoyantlanchor. lines. to .hold: said floating structure f :against;disp1acement.- under 1 external. forces will become 'apparentirom the iforegoingfiescription. :Ho wever, all

matemwin occur tofthos'e-sskilled :inzthe cart. Like-1 ofi-hingedly interconnected closed elongated tubular; seg- ;.ments ofisubstantiallyineutral buoyancy extendingzirom "said-buoyancy platform" outwardly'toanchor means at the underwater bottom and .'disposed with the individual segments forming; said lines aextending inzisubstantially ;a-st r aight linea relationship through'their-ilength from .said anchor meansxto said .buoyancy ;platform,,means 'such 'modific'ations 'orcchangesi'in .th'e method-:of. estabextendingvertically abovessaid buoyancy plat-form,- a; work itpla'tform supported l on 2 said columns; lthetbuoyancy; .of :saidtbuoyantl: platform being; substantially zincexcess: of

' athej totalweight of. said workplatformiandithe working weight to be applied. to .said platform; a: plurality-Jot ver- :tical; anchor lines eachzcomprising' a'fplurality othin'gedly interconnected c1osed:pipe sections of: substantiallyneu- .:.tral' buoyancy extending. vertically doWn'WardLfromsaid buoyancy platform.and adaptedto engagelassimilar plurralitylof anchor means embedded in thevunderwater. bottom,-:-positive tension means. connectingzs'aid buoyancy 'zplatform and said verticalz-anchor. lines :for -pulling said buoyancy platform a alongmthe length of said vertical an- 1 char flines ito hold said :buoyancy' platform at -aneleval gtion:belowatheunormal:wave and tideaction, a plurality of'iateml anchorlines -each comprisingaplurality interconnecting atv least twoof saidElatet-alenchor lines,

- said interconnecting means including flexiblecforce transomitting means connectingTthe-outwardl-y disposed ends of 7 two lateral anchorlines and;pulley means positioned'on said anchor means, which pulley,meanssaidiforce transvrnitting connectingvmeans operably engages t0:;transmit .forcesapplied to and :reactive 1 forces generated: within oneofi: said "two connected .lateral. lines: to .the other of said interconnected,lateral lines, and i. means ,for applying a predeterminable tension to-said lateralranchor linesjto 7 restrain said=work {platform from v lateral movement, [said tensionapplying means comprising connectingnmeans be- 20 tween-,isaid;buoyancy platform: andiat least one, of said interconnected, lateral-anchor .lines includin'g means for applyingapredeterrnined tensile. force to said lateralilines and. .means .for maintainingpsaid .applied. force. 

